The invention relates to a grid which includes comb elements which absorb electromagnetic radiation and are intended to form a grid.
Grids of this kind are used in the X-ray technique as anti-scatter grids for absorbing scattered radiation arising in the tissue of the patient before the characteristic X-ray signal, produced by the varying attenuation properties of the tissue examined, is incident on the X-ray detector.
U.S. Pat. No. 5,099,134 discloses a collimator (anti-scatter grid) and a method of manufacturing such a collimator. The collimator is formed by a frame which absorbs X-rays and in which first and second partition plates are arranged. Each of the partition plates is provided with slits which extend in the longitudinal direction of the partition plate and enable the first partition plates to be inserted into the second partition plates at the appropriate angle. The inner sides of the rectangular frame are provided with slits which serve to receive the respective ends of the partition plates.
The complexity of the partition plates imposes given limits on the manufacture of such anti-scatter grids. It has been found that the manufacture of anti-scatter grids of large dimensions, for example as used for large-area detectors, is difficult, because bow of the large partition plates impedes simple and correct meshing of the slits of the partition plates.
Large-area anti-scatter grids are used, for example for multi-line CT (Computed Tomography) apparatus. The length of the detector is then very large. The X-rays emitted by an X-ray source in computed tomography traverse the patient and are attenuated in conformity with the varying thickness and chemical composition of the tissue or bone to be examined. The X-ray signal at the same time is subject to scattered radiation. In order to reduce such scattered radiation which falsifies the primary X-ray image to be formed, the X-rays traverse an anti-scatter grid which is focused onto the focal spot of the radiation source. It is thus achieved that only the X-ray quanta that are characteristic of the attenuation of the irradiated object are detected during the detection of the X-ray quanta.
The construction of CT examination apparatus is such that the radiation source is mounted opposite the detector on a gantry which rotates about the patient, the patient being slowly displaced by means of a flat bed. Vibrations of the gantry, being transferred to the anti-scatter grid and the X-ray detector, have a negative effect on the quality of the image to be formed. Such negative effects cannot be imitated, so that such image falsifying effects can be reduced to a limited extent only during later image processing.
In order to achieve a fast X-ray procedure, the width of the X-ray beam is increased. A larger surface of the object to be examined is thus scanned in a single scan, and hence also a larger volume. However, this has the drawback that the scattered radiation component increases. In order to reduce such an increasing scattered radiation component, the height of the anti-scatter grid is increased. Known anti-scatter grids, however, are not sturdy enough for this purpose.
Therefore, it is an object of the invention to provide an anti-scatter grid which aims to reduce scattered radiation and can be simply manufactured with a sturdiness which also suffices for large-area anti-scatter grids.
This object is achieved in that comb lamellae extend transversely of an associated comb base surface which supports the comb lamellae.
An anti-scatter grid is arranged over the X-ray detector in such a manner that the primary X-rays are incident, via the grid, on a respective detector element arranged therebelow.
The anti-scatter grid consists of a plurality of comb elements which absorb X-rays, are provided with comb structures and are fixed in a frame. The comb elements preferably have a rectangular basic shape and comprise comb lamellae which extend transversely of the surface of a base plate and to the comb base surface formed thereby. These comb lamellae form the comb structure. The comb lamellae are focused onto the focal spot of the radiation source, with the result that the distance between the comb lamellae at the top of the comb element is less than that at the lower side. A plurality of such comb elements is arranged in such a manner that the comb lamellae which extend transversely of the comb base surface border or abut against the nearest comb element by way of the associated comb base surface. This results in a two-dimensional grid structure. The distance between the comb lamellae and the depth of the comb lamellae defines the resolution of the anti-scatter grid. The grid openings of this two-dimensional grid are oriented in the direction of the incident X-rays.
The sides of the individual comb elements are secured in the frame by way of grooves. The number of comb elements to be linked is determined by the dimensions of the X-ray detector used. In the case of CT apparatus, the length of the X-ray detector usually mounts to a number of times its width. It has been found that the comb elements advantageously are so rugged and stable that a large number of comb elements can be arranged in a frame, thus forming a large-area anti-scatter grid which covers a large-area X-ray detector.
In the case of X-ray exposures the X-rays which are characteristic of the region examined are converted in an X-ray detector, for example, into light which itself is either read out by a light-sensitive sensor or is used to expose a film accordingly.
In the case of digital X-ray detectors the image information is read out by sensors. In these discrete exposures it is important that the X-ray quanta of a relevant examination zone to be imaged on a pixel are converted only in the associated detector element and are detected only by the corresponding sensor which is situated therebelow the X-ray quanta that are characteristic of the examination zone corresponding to the resolution of the detector reach the associated detector element directly via the relevant grid opening of the anti-scatter grid. The X-ray quanta that are characteristic of an examination zone corresponding to the resolution of the detector are conducted directly to the associated detector element via the corresponding grid opening in the anti-scatter grid. The radiation scattered in the transverse direction is absorbed by the grid structure of the anti-scatter grid.
The anti-scatter grid in a further embodiment is made of comb elements having a double comb structure and plane lamellae. The comb elements have comb lamellae which extend in the direction transversely of the base plate on both sides of the base plate. The comb lamellae of these double comb elements extend transversely of the two comb base surfaces to both sides of the base plate. For the anti-scatter grid a double comb element and a plane lamella are linked each time in an alternating fashion. This again results in a grid. The double comb elements and the lamellae are retained by the frame.
The comb lamellae of the comb elements are oriented so as to be focussed onto the focal spot of the radiation source. The X-rays are incident on the anti-scatter grid at a given angle. Because the direct X-rays should pass the anti-scatter grid without impediment, the orientation of the grid must be adapted to the radiation angle. To this end, the distances between the comb lamellae at the top of the comb elements are smaller than the distances between the comb lamellae at the bottom of the comb elements.
Moreover, in the case of curved X-ray detectors it is also necessary to adapt the anti-scatter grid to the curvature of the X-ray detector. To this end, the depth of the comb lamellae increases towards the lower side of the comb element, so that a curvature corresponding to the curvature of the X-ray detector is obtained upon assembly of a plurality of comb elements.
The frame in which the comb elements are secured is adapted to the shape of the X-ray detector. Grooves are provided on the inner sides of the frame. The thickness of the grooves corresponds to the wall thickness of the comb elements, so that they are retained by the shape of the grooves. Additionally, the comb elements can be glued in said grooves.
The object according to the invention is also achieved by means of a detector with a grid for the absorbtion of X-rays.
Furthermore, the object of the invention is also achieved by means of an X-ray apparatus which includes a grid for the absorption of X-rays which is arranged in front of the detector.
The object according to the invention is also achieved by means of a method of manufacturing a grid which includes comb elements which absorb electromagnetic radiation, the comb elements with comb lamellae extending perpendicularly to an associated comb base surface supporting the comb lamellae being arranged in such a manner that they form a two-dimensional grid.
Embodiments of the invention will be described in detail hereinafter with reference to the drawing. Therein: